FIG. 7a shows a pressure gauge according to the prior art disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-310823. To measure a pressure of a fluid flowing inside a conduit line 101 as shown in FIG. 7a, a pressure gauge 102 is interposed into the conduit line 101. The pressure gauge 102 is fitted by screwing it into a fitting hole 105 of a coupling 104 to which the conduit line 101 is connected. The pressure gauge 102 detects the pressure of the fluid flowing inside a flow passage 106 formed in the coupling 104 and communicated with the conduit line 101 and displays the pressure of the fluid by a display portion 108 having a pointer 107.
FIG. 7b is a partial enlarged view of the pressure gauge shown in FIG. 7a. As shown in FIG. 7b, the pressure gauge 102 includes a fitting portion 111 having a male screw to be screwed into the fitting hole 105 of the coupling 104 and a measurement portion 112 fixed to this fitting portion 111. A pressure detection passage 111a is formed in the fitting portion 111 and the pressure of the fluid from the pressure detection passage 111a acts on a diaphragm 113. The diaphragm 113 is shaped into a bottomed cylinder shape and its peripheral surface is an expandable-contractible bellows portion 114 shaped into a bellows shape. An operation rod 115 of the measurement portion 112 is fitted into the diaphragm 113 from the upper side thereof. In other words, in the pressure gauges 102 of this kind, the contractible diaphragm 113 is disposed in such a fashion that the fluid does not directly contact the operation rod 115 of the measurement portion 112 and the pressure of the fluid is transmitted to the operation rod 115 via the diaphragm 113.
In the pressure gauge 102 disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-310823, a protective cap 119 is disposed for covering around an end portion of the operation rod 115. The protective cap 119 is formed of a corrosion-resistant material and prevents corrosion of an internal mechanism of the pressure gauge 102 by a chemical solution when the pressure of the conduit line 101 through which the chemical solution flows is measured.
The pressure of the fluid flowing inside the conduit line 101 is transmitted from the pressure detection passage 111a of the fitting portion 111 to the diaphragm 113, and as a result, the diaphragm 113 is compressed upward by the pressure of the fluid and the operation rod 115 fitted into the diaphragm 113 is lifted up. With the movement of this operation rod 115, the pointer 107 of the display portion 108 of the measurement portion 112 is pivoted to indicate the pressure.
When a valve (not shown in the drawing) for opening and closing the conduit line is disposed in the conduit line immediately downstream from the pressure gauge, for example, the pressure drastically increases immediately before the valve if the fluid inside the conduit line 101 is abruptly stopped by the valve, and the pressure drastically drops if the valve is abruptly released, which results in what is known as a “water hammer”. Even when such a water hammer occurs, the pressure gauge linearly responds in accordance with the magnitude of the water hammer and consequently, the internal mechanism portion of the pressure gauge such as the diaphragm and the operation rod may be broken owing to the force of the water hammer.
The pressure gauge disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-310823 avoids the influences of the chemical solution by using the material having high chemical resistance as described above, but cannot prevent a water hammer from occurring. Therefore, breakage of the pressure gauge is likewise a matter of concern.
The present invention is completed in view of the circumstances described above and is directed to provide a pressure gauge capable of suppressing breakage of an internal mechanism resulting from a pressure change phenomenon inside a conduit line such as a water hammer, and a pressure gauge assembly having such a pressure gauge.